In the art of printing, a variety of substrates must be coated with glossy finishes. These substrates are typically sheets of paper or cardboard; however, the term "sheet" also includes any other flat surface or substrate such as fabric, boxes, magazines, newspaper inserts, and bonding thermoplastic substrates.
There are basically two broad classes of coatings which are used in many conventional printing processes. The first class of coatings may be referred to as acrylic polymers. Acrylic polymer coatings are typically soluble in water, or in a solvent, and are suspended or dissolved in a solution before being applied to a substrate. Once applied to the substrate, acrylic polymer coatings may be cured by applying heat to evaporate the water or solvent base. The second class of coatings used in many conventional printing operations may be referred to as ultra-violet polymers. These coatings are cured using ultra-violet light to bond, or polymerize, monomer molecules in the coating.
Ultra-violet polymer coatings are generally preferred in many conventional coating processes for their superior aesthetic properties. Ultra-violet coatings tend to produce glossier finishes than acrylic coatings due to their flow characteristics and the larger amount of solid material that remains on the substrate after curing. In order to achieve comparably glossy finishes with acrylic coatings, more coating material must be applied to the substrate which then decreases the smoothness of the acrylic finish.
Although ultra-violet polymer finishes may be preferred in conventional coating processes for aesthetic reasons, ultra-violet coating processes have significant drawbacks. For example, the ozone which is produced as a by-product of curing with ultra-violet radiation has been found to threaten air quality. In addition, paper products which are coated with ultra-violet polymers are often difficult to recycle. Water-based acrylic coatings, on the other hand, are much easier to recycle and do not require ultra-violet radiation for curing. Consequently, there is a long felt and continuing need to improve the aesthetic qualities of sheets that are coated with acrylic polymers, and other coatings, in order to avoid the environmental problems associated with ultra-violet polymer coating materials.
Many recyclable surface finishes, including acrylic polymers, are activated by heat and pressure. Such coating materials are typically applied to sheets in liquid form and then allowed to air dry. The coated and dried sheets are then pressed against a heated, flat surface for a certain period of time and then allowed to gradually cool before being stripped from the surface. In order to create an acceptable finish on the sheet, the texture, temperature, and pressure of the compression surface along with the heating time and pressure must be tightly controlled during the finishing process. There is also a need to automate and increase the speed of the process in order to maximize the number of sheets which may be finished during any give time period while still producing a high quality surface finish.
A known sheet calendering device has used one continuous, mirror-finished belt arranged between two cylinders. Sheets are fed onto the belt at one end of the process and then heated, with steam, and compressed between the belt and a third cylinder. The steam-heated sheets are then cooled with a liquid coolant prior to their delivery at the other end of the process. One disadvantage of using such continuous belt arrangements for sheet calendering is that the entire belt must be replaced when even just a small portion of the belt is damaged. It is also difficult to maintain sufficient tension in the belt, particularly in long belts, to ensure proper belt tracking and minimize vibrations at high speeds. Furthermore, steam heating systems are complicated to build and difficult to operate and maintain. And finally, it is difficult to position sheets on the belt at the feed end of the apparatus and to remove the cooled sheet from the belt at the delivery end of the apparatus. Although the effects of many of these problems on surface finish quality can be minimized by using ultra-violet polymer coatings, acrylic coatings are often preferred for environmental reasons. Consequently, it is the object of the present invention to provide a high speed sheet calendering apparatus which overcomes these and other disadvantages of conventional sheet calendering technology.